Egg drop syndrome is caused by an adenovirus infection that affects the reproductive organs of the chicken.

The combination of a sudden fall in egg production, associated with thin-shelled, soft-shelled and shell-less eggs in a flock of apparently healthy chickens is almost diagnostic.  The first signs are usually a loss of shell strength and pigmentation.  When established the condition is seen as a failure to reach production targets, with less apparent eggshell changes.

Classical egg drop syndrome probably was due to contamination of a vaccine for Marek’s disease. Basic breeding stock was infected and the virus was transmitted vertically through the egg.  The virus often remained latent until the chicken reached sexual maturity, when it was excreted in the eggs and droppings to infect susceptible contacts.  Arising from the classical form, endemic egg drop syndrome is usually seen in commercial egg producers.

Flocks become infected at any stage of lay. Contaminated egg collection trays are one of the main forms of horizontal transmission.  Outbreaks are often associated with a common egg packing station.  Sporadic egg drop syndrome has been recognised in isolated flocks.  It appears to be due either to contact with domestic ducks or geese, or more often, to water contaminated with wildfowl droppings.  The risk is that these introductions could become endemic.

Egg drop syndrome can appear similar to infectious bronchitis virus (IBV) and it is important to have a diagnosis performed by your veterinarian.